Synthesis and Chemistry of Zwitterionic Tantala-3-boratacyclopentenes: Olefin-like Reactivity of a Borataalkene Ligand

The compounds

, where Cp‘ = C5H5, a series, and C5H4Me, b
series, are generated via treatment of

with HB(C6F5)2. When allowed to undergo
irreversible methane loss in the presence of an excess of the sterically modest alkynes 2-butyne or
phenylacetylene, the putative intermediates

1a and 1b are trapped as the tantala-3-boratacyclopentene compounds 2 and 3, respectively. In these complexes, the alkyne and borataalkene
ligands have reductively coupled at the d2 tantalum center. For the unsymmetrical alkyne, a kinetic product
resulting from coupling in the opposite regiochemical sense is observed; the thermodynamic products 3-t
incorporate the phenyl group in the α position of the tantalaboratacyclic ring. Two of these compounds (2b
and 3b-t) were characterized crystallographically. For bulkier alkynes (diphenylacetylene, 1-phenyl-1-propyne, and 3-hexyne), intermediates with similar spectroscopic properties to the tantala-3-boratacyclopentenes were observed, but the ultimate products were the vinylalkylidene compounds 5-(R,R‘). Compound
5-(Ph,Me) was characterized crystallographically, and it was found that the vinylalkylidene binds to the
metal in an η1-bonding mode, with the tantalum center receiving further ligation through a hydridoborate
moiety. Mechanistic studies suggest that these products arise via retrocyclization of the tantala-3-boratacyclopentenes formed kinetically. These studies represent the first studies concerning the reactivity
of a borataalkene ligand at a transition metal center and show that it can behave in an “olefin-like” manner,
despite having a more flexible array of bonding modes available to it than an olefin.